Near-field scanning optical microscopes (“NSOMs”) operate by scanning an optical probe (“probe”) over a sample. Depending on the mode of operation of an NSOM, the probe may illuminate or collect light, or both. The probe passes light through an aperture smaller than the wavelength of the light. The probe and/or sample are scanned such that the aperture passes over the area to be imaged. An image so constructed occurs on a line-by-line or point-by-point basis. Typical NSOMs use piezoelectric transducers to perform the scanning motions. The spatial resolution achievable by an NSOM is not limited by the wavelength of the light, as in standard microscopy, but rather by the dimension of the aperture through which the light passes (i.e., a smaller aperture produces a higher resolution image) and by the spacing of the points or lines that make up the image.
An NSOM may also act as a light source to produce subwavelength images in photoresist. A substrate is coated with photoresist and placed on an NSOM stage. The NSOM probe and/or the substrate are scanned to move the probe's aperture over an area of photoresist to be imaged, to expose the photoresist line-by-line or point-by-point. Photoresist image resolution depends upon the dimension of the aperture and on the spacing of the points or lines during scan.
Existing electron beam tools for direct writing exposure of photoresist on a substrate expose the substrate to vacuum conditions and high energy electrons. Imaging of surface features (i.e., alignment features) in electron beam tools also unavoidably exposes photoresist over such features. Images produced by existing conventional lithography tools such as mask projection or contact aligners expose entire regions simultaneously through photomasks. The images produced by conventional lithography tools are also subject to the effects of diffraction.